from a #define to a run-time option '-e'

Man page was updated to reflect new option

* Wrote max(date) and min(date) aggregates
* Wrote operator "-" for date; date - date yields number of days
  difference
* Wrote operator+(date,int) and operator-(date,int); the int is the
  number of days.  Each operator returns a new date.


By: Tom Tromey <tromey@creche.cygnus.com>

Goals: reduce the difficulty of porting from platform to platform,
       release to release, but moving as much as possible into config.h

- submitted by: Massimo Dal Zotto <dz@cs.unitn.it>